A motion detector is a device that detects moving objects, particularly people. A motion detector is often integrated as a component of a system that automatically performs a task or alerts a user of motion in an area. Motion detectors can form an important part of a security system, automated lighting control system, home control system, and other systems.
Motion detectors usually employ pyroelectric materials to detect the movement of people in a room. A pyroelectric material generates a signal if the incoming heat radiation (from a heat source such as a person's body) changes. Mathematically, the pyroelectric detector generates an electrical signal that follows the time derivative of the incoming heat flux. Thus, if a person enters or leaves the field-of-view (FOV) of the detector, the heat flux changes and a respective signal is generated. The height of the signal is dependent on the temperature of the heat source and the so-called filling factor of the field of view.
The higher the temperature of the source and the more the source fills the FOV of the detector, the higher the resulting signal. This signal will exist only for a limited time after a heat flux change, and thus if the heat flux remains constant then no signal is generated. As such it is impossible to detect the presence or absence of a motionless warm object such as a person standing still or having left the sensor area.
The use of two differentially arranged thermal DC sensors is described, for example, by U.S. Pat. No. 4,722,612, patent to Junkert et al., directed to infrared thermometers for minimizing errors associated with ambient temperature gradients using a dual thermopile where the second thermopile is used as a compensation element. In some cases the second element is used to thermally and electrically compensate thermal drift for the main sensor, and as such the compensating device is typically blinded from the incoming radiation and only provides ambient temperature compensation.
In thermopile-based flow meters, two identical sensors may be subjected to the heat from a local heater, but on opposite sides of the heater, so that a flow of gas over the sensor-heater-sensor combination may affect the heat flow and give an output, but with inherent thermal drift compensation. Similarly pyro devices using lead zirconate titanate (PZT) may use a similar configuration to prevent DC drift but each pyro is usually illuminated by different light at different times, for example, by a lensing arrangement in front of the sensor, to produce an AC signal but suppressing the DC signal. In a typical implementation, the technology disclosed herein addresses one or more of the above mentioned deficiencies.